LF: Fwd: 500kHz JT4A

[Andy Talbot <andy.g4jnt@googlemail.com>]

Dear Andy,

I tried to send this to the LF reflector, but I am away from home at the moment, and it does not seem to want to accept mail from me - perhaps you could forward it...

Dear Rik, Andy,

I have been using a push-pull class D PA in my "EER transverter" for various FSK (and linear) modes over the last year or so. To get a symmetrical square-wave drive from the low-level (basically sinusoidal) 500k input, the input signal is squared up by a limiting amplifier, to a TTL level, then drive to one gate is simply inverted. To get a 50/50 square wave drive, it is important that the limiter has symmetrical limiting thresholds - the circuit I used can be seen in the EER Transverter v3 article on the UK_500kHz group files download page at http://groups.google.co.uk/group/uk500khz/files. It is also important that the input to the limiter has a symmetrical waveform - but this is easily achieved by simple band-pass filtering, a single tuned circuit with a loaded Q of, say, 5 will produce a nice sine wave from a fairly horrible input waveform.

Cheers, Jim Moritz
73 de M0BMU

--- On Wed, 27/1/10, Andy Talbot <andy.g4jnt@googlemail.com> wrote:

> From: Andy Talbot <andy.g4jnt@googlemail.com>
> Subject: Re: LF: RE: 500 kHz JT4A
> To: rsgb_lf_group@blacksheep.org
> Date: Wednesday, 27 January, 2010, 21:56
> I was looking at driving push-pull
> transmitters from fundamental drive signals quite
> recently.   
> 
> How about a frequency doubler?    The output
> doesn't have to have 1:1 mark-space ratio as it only
> clocks a flip-flop.   And simple doubler can consists of
> a delay line (CR network with around 0.5us time constant)
> followed by a squarer to give a resulting waveform shifted
> in phase from the 500kHz drive signal.  Then apply this
> one,  along with the original to a two input XOR gate
> whose output will then consists of a 1MHz train suitable for
> dividing by 2 for a perfect 1:1 squarewave drive.
>
>